The company says plant emergency sirens sounded but the small fire was quickly put out and there were no injuries.
Spokesman Jon Harding says plant personnel responded at 4:45pm Wednesday and the all clear was issued at 5:18pm.
He says the community and the Ministry of the Environment were notified.
The site is currently undergoing a period of planned maintenance but Harding says the fire was not related to the work that is underway.
Sarnia fire responded as a precaution. Chief John Kingyens says the fire occurred when a breaker in a substation tripped open and Imperial Oil staff put it out with a portable extinguisher.
Black smoke could be seen coming from a flare stack because the breaker caused a compressor in a process unit to shut down.
Fire officials say there were no indications of volatile organic compounds during downwind air monitoring.
Harding says the hydrogen purification system was affected causing increased flaring.